In one study, researchers at the University of California asked one group of students to visualize themselves getting a great grade on an exam. They asked another group to visualize themselves studying for the exam.
The students who visualized themselves getting a good grade scored lower on the test than the students who visualized themselves studying. Students who visualized themselves studying put more time into preparing for the test and ultimately, they scored much better.
That's just one example. There are a multitude of studies that show athletes, students, and musicians perform worse when they visualize themselves succeeding, as opposed to visualizing themselves going through the steps it takes to succeed.